Nonwoven fabrics are well-suited for applications which require a low cost fibrous web. Examples are disposable articles such as polishing or washing cloths, cast paddings and facing layers for fibrous mat products.
Nonwoven fabrics are normally produced from a web of loosely associated fibers that are subjected to a fiber rearranging method to entagle and mechanically interlock the fibers into a unitary reticular network. The fiber rearrangement is achieved under the effect of fluid forces applied to the fibers through a fluid permeable web confining and supporting structures comprising a rigid apertured member with a predetermined pattern of fluid passages, and a flexible foraminous sheet disposed in a face-to-face relationship to the apertured member. In one form of construction, the rigid apertured member is a rotating hollow drum and the flexible foraminous sheet is an endless screen belt in overlapping relationship with the hollow drum and advancing therewith. The web of loosely associated fibers which forms the starting material of the nonwoven fabric production method is confined between the drum and the screen belt and is advanced through a fluid stream creating the entagling forces on the fibers.
The so-called "Rosebud" nonwoven fabric production method requires that the fluid stream be located outside the hollow drum, the fluid particles impinging on the fibers through the screen belt. In operation, the fibers are drawn by the fluid mass flowing out of the apertured hollow drum, into the fluid passages thereof, and they are mechanically interlocked and entagled in protuberant packings which are interconnected by flat fiber bundles extending over the land areas of the drum. The resulting nonwoven fabric has a three-dimensional structure presenting a knobby side containing the apexes of the fiber packings, and a flat and smoother side containing the base portions of the fiber packings and the interconnecting bundles.
In a variant of the Rosebud method, known as the "Keybak" method, the direction of the fluid stream is reversed, whereby the fluid particles reach the fibers by passing through the fluid passages on the drum. In contrast to the Rosebud method, the fibers are packed together on the land areas of the drum forming a network with clear holes arranged into a pattern corresponding to the pattern of fluid passages on the hollow drum.
For a wide range of applications, nonwoven fabrics having superior resistance characteristics are required. Basically, the resistance or durability of a nonwoven fibrous web depends on the degree of fiber entanglement achieved during the fiber rearranging process. When the fibers are tightly interlocked, they form a dense and tenacious network which is highly resistant to forces tending to destroy the web integrity, such as tear forces for example. In contrast, a web constituted by loosely associated fibers is substantially less resistant because, at the fiber level, the network of the web lacks cohesion.
In conventional nonwoven fabric production methods, a certain increase in the degree of fiber entanglement may be achieved at the fiber rearranging stage by increasing the fluid supply pressure of the stream in order to augment the intensity of the fluid forces acting on the fibers. However, there are disadvantages and inherent limits in increasing the fluid supply pressure which considerably offset any advantage that may be gained in terms of higher fiber entanglement. Traditional production methods already require fairly high fluid supply pressures and a further pressure increase creates considerable strain on the equipment which translates into an increase of the fabric manufacturing cost. In addition, regardless of cost considerations, the fluid supply pressure cannot be indefinitely increased as beyond a certain point, a destructive condition known as "flooding" occurs which is defined as a loss of web identity resulting from the application of fluid forces to the fiber which are too intense.
It is also known from the prior art to apply a binder substance to the fibers of the fabric subsequently to the fiber rearranging step, in order to increase the fabric resistance. The binder substance, when cured, establishes a bond between adjacent fibers and prevents them to move one relative to the other. Accordingly, the tenacity of the fabric will increase because of the reduction in the inter-fiber displacement when destructive forces act on the fabric.
Although a binder can effectively increase the resistance of a nonwoven fabric, for cost considerations, it cannot be considered as an ideal solution. Fundamentally, the objective of any nonwoven fabric production method is to turn out the least expensive product, therefore, it is desirable to eliminate or at least reduce as much as possible the binder application.